(a) Field of the Invention
This disclosure relates to a hollow fiber, a dope solution composition for forming a hollow fiber, and a method of preparing a hollow fiber using the same.
(b) Description of the Related Art
Membranes must satisfy the requirements of superior thermal, chemical and mechanical stability, high permeability and high selectivity so that they can be commercialized and then applied to a variety of industries. The term “permeability” used herein is defined as a rate at which a substance permeates through a membrane. The term “selectivity” used herein is defined as a permeation ratio between two different gas components.
Based on the separation performance, membranes may be classified into reverse osmosis membranes, ultrafiltration membranes, microfiltration membranes, gas separation membranes, etc. Based on the shape, membranes may be largely classified into flat sheet membranes, spiral-wound membranes, composite membranes and hollow fiber membranes. Of these, asymmetric hollow fiber membranes have the largest membrane areas per unit volume and are thus generally used as gas separation membranes.
A process for separating a specific gas component from various ingredients constituting a gas mixture is greatly important. This gas separation process generally employs a membrane process, a pressure swing adsorption process, a cryogenic process and the like. Of these, the pressure swing adsorption process and the cryogenic process are generalized techniques, design and operations methods of which have already been developed, and are now in widespread use. On the other hand, gas separation using the membrane process has a relatively short history.
The gas separation membrane is used to separate and concentrate various gases. e.g. hydrogen (H2), helium (He), nitrogen (N2), oxygen (O2), carbon monoxide (CO), carbon dioxide (CO2), water vapor (H2O), ammonia (NH3), sulfur compounds (SOx) and light hydrocarbon gases such as methane (CH4), ethane (C2H6), ethylene (C2H4), propane (C3H8), propylene (C3H6), butane (C4H10), butylene (C4H8). Gas separation may be used in the fields including separation of oxygen or nitrogen present in air, removal of moisture present in compressed air and the like.
The principle for the gas separation membranes is based on the difference in permeability between respective components constituting a mixture of two or more gases. The gas separation involves a solution-diffusion process, in which a gas mixture comes in contact with a surface of a membrane and at least one component thereof is selectively dissolved. Inside the membrane, selective diffusion occurs. The gas component which permeates the membrane is more rapid than at least one of other components. Gas components having a relatively low permeability pass through the membrane at a speed lower than at least one component. Based upon such a principle, the gas mixture is divided into two flows, i.e., a selectively permeated gas-containing flow and a non-permeated gas-containing flow. Accordingly, in order to suitably separate gas mixtures, there is a demand for techniques to select a membrane material having high perm-selectivity to a specific gas ingredient and to control the material to have a structure capable of exhibiting sufficient permeance.
In order to selectively separate gases and concentrate the same through the membrane separation, the membrane must generally have an asymmetric structure comprising a dense selective-separation layer arranged on the surface of the membrane and a porous supporter with a minimum permeation resistance arranged on the bottom of the membrane. One membrane property, i.e., selectivity, is determined depending upon the structure of the selective-separation layer. Another membrane property, i.e., permeability, depends on the thickness of the selective-separation layer and the porosity level of the lower structure, i.e., the porous supporter of the asymmetric membrane. Furthermore, to selectively separate a mixture of gases, the separation layer must be free from surface defects and have a fine pore size.
Since a system using a gas separation membrane module was developed in 1977 by the Monsanto Company under the trade name “Prism”, gas separation processes using polymer membranes has been first available commercially. The gas separation process has shown a gradual increase in annual gas separation market share due to low energy consumption and low installation cost, as compared to conventional methods.
Since a cellulose acetate semi-permeation membrane having an asymmetric structure as disclosed in U.S. Pat. No. 3,133,132 was developed, a great deal of research has been conducted on polymeric membranes and various polymers are being prepared into hollow fibers using phase inversion methods.
General methods for preparing asymmetric hollow fiber membranes using phase-inversion are wet-spinning and dry-jet-wet spinning. A representative hollow fiber preparation process using dry-jet-wet spinning comprises the following four steps, (1) spinning hollow fibers with a polymeric dope solution, (2) bringing the hollow fibers into contact with air to evaporate volatile ingredients therefrom, (3) precipitating the resulting fibers in a coagulation bath, and (4) subjecting the fibers to post-treatment including washing, drying and the like.
Organic polymers such as polysulfones, polycarbonates, polypyrrolones, polyarylates, cellulose acetates and polyimides are widely used as hollow fiber membrane materials for gas separation. Various attempts have been made to impart permeability and selectivity for a specific gas to polyimide membranes having superior chemical and thermal stability among these polymer materials for gas separation. However, in general polymeric membrane, permeability and selectivity are inversely proportional.
For example. U.S. Pat. No. 4,880,442 discloses polyimide membranes wherein a large free volume is imparted to polymeric chains and permeability is improved using non-rigid anhydrides. Furthermore, U.S. Pat. No. 4,717,393 discloses crosslinked polyimide membranes exhibiting high gas selectivity and superior stability, as compared to conventional polyimide gas separation membranes. In addition, U.S. Pat. Nos. 4,851,505 and 4,912,197 disclose polyimide gas separation membranes capable of reducing the difficulty of polymer processing due to superior solubility in generally-used solvents. In addition, PCT Publication No. WO 2005/007277 discloses defect-free asymmetric membranes comprising polyimide and another polymer selected from the group consisting of polyvinylpyrrolidones, sulfonated polyetheretherketones and mixtures thereof.
However, polymeric materials having membrane performance available commercially for use in gas separation (in the case of air separation, oxygen permeability is 1 Barrer or higher, and oxygen/nitrogen selectivity is 6.0 or higher) are limited to only a few types. This is because there is considerable limitation in improving polymeric structures, and great compatibility between permeability and selectivity makes it difficult to obtain separation and permeation capabilities beyond a predetermined upperbound.
Furthermore, conventional polymeric membrane materials have a limitation of permeation and separation properties and disadvantages in that they undergo decomposition and aging upon a long-term exposure to high pressure and high temperature processes or to gas mixtures containing hydrocarbon, aromatic and polar solvents, thus causing a considerable decrease in inherent membrane performance. Due to these problems, in spite of their high economic value, gas separation processes are utilized in considerably limited applications to date.
Accordingly, there is an increasing demand for development of polymeric materials capable of achieving both high permeability and superior selectivity, and novel gas separation membranes using the same.
In accordance with such demand, a great deal of research has been conducted to modify polymers into ideal structures that exhibit superior gas permeability and selectivity, and have a desired pore size.